Roles of plasma proteins in the formation of silicotic nodules in rats

The contribution of plasma protein(s) to the stabilization of fibroids formed in rat lungs exposed to acute silica dust inhalation was examined. Antibodies against component proteins of the nodules remaining insoluble in 2% SDS, 10M urea and 40 mM sulfhydryl reagents under prolonged boiling conditions were raised in rabbits and used to capture plasma proteins, which were identified by 2D-gel electrophoresis and MALDI-TOF analysis. The silica particles were encapsulated with extracellular protein composites whose amino acid compositions showed high levels of alanine, i.e., above those of glycine and proline, a building block of collagen. Antibody-captured plasma proteins showed the dominant presences of fibrinogen, albumin, and prealbumin (transthyretin), and other minor proteins, which included alpha-1-protease inhibitor, contraspin-like protease inhibitor, cathepsin B, etc. The presence of the N(epsilon)-(gamma-glutamyl) lysine isopeptide bond in the nodules was evidenced by direct chemical methods and by immunoreactivity for anti-isopeptide bonds. Immunostaining of affected lung tissue and of the fibroid regions showed elevated levels of transglutaminase (TGase) E and plasma factor XIII (F-XIII), but showed no reactivity towards other TGases. These findings suggest that the silica encapsulated nodules are a mixture of extracellular proteins that include collagen type I, fibrin and transthyretin, which is stabilized by TGase catalyzed crosslinking between plasma and extracellular proteins during fibrosis to eventually form insoluble nodules.